The invention relates to a process and apparatus for determining the consumption of a raw material in a processing machine and, more particularly, for determining the consumption of a fluid plastics material in a plurality of extrusion or injection molding machines, wherein the raw material is stored in a central storage container or silo and supplied to the processing machine or machines by a conveyor system, e.g. a vacuum conveyor system.
For a batch-related re-calculation a gravimetric determination of the amount of raw material consumed in the individual processing machines is often desired. A re-calculation is required e.g. to consider unavoidable losses in raw material at the start up of the machine operation after removing an operating disturbance or the like. The quantity of raw material consumed under such circumstances can be so significant that it is no longer negligible in relation to the weight of the manufactured product. It is further known that with large material throughput stock deviations frequently occur whereby it was up to now difficult and/or impossible to correlate them with the individual processing machines.
It is known for determining the raw material consumption to continuously weigh the amount of raw material which is contained in a receiving vessel mounted on the machine, e.g. a separator or a cyclone unit, i.e. directly at the processing machine. This kind of determination of consumption is not only impaired by the unvoidable vibrations in the vicinity of the processing machine, but it is further connected with considerable costs, since a complete gravimetric determination system must be provided at each processing machine. It is also known to determine gravimetrically the raw material consumption in the vicinity of a central raw material storage container from which a plurality of processing machines can be supplied, in that the weight of the central storage container with the raw material contained therein is continuously weighed, e.g. by means of a pressure cell or the like the storage container is superimposed. A severe disadvantage of this kind of weight determination is its inaccuracy, because generally the charges for an individual processing machine are some kilograms only, while the total weight of a storage container can be of the order of several tons.